A magnetic resonance imaging diagnosis, i.e., MRI, is receiving plenty of attention, since any cross-sectional image of human body can be taken using magnetism and radiowave. In a MRI, a contrast agent for MRI, such as Magnevist (registered trademark, generic name: meglumine gadopentetate), is sometimes used in order to obtain a clearer and more detailed image.
Magnevist, by which the contrast of an image can be improved, has a highly toxic gadolinium ion. In the chemical structure thereof, the gadolinium ion is chelated by DTPA: DiethyleneTriaminePentaacetic Acid as follows so that the toxicity is neutralized.

Magnevist rapidly diffuses out into a living body after administration, and then is eliminated in urine. Such a property is preferable in terms of safety; however, a retention property in blood for a longer time and specificity to an intended organ are also needed for diagnosis.
For the above-described property, it can be thought that a substituent for improving a retention property in blood and specificity to an intended organ is bound to DTPA through the carboxy groups thereof (Non-patent Document 1). However, such a modification increases the probability of spread of naked gadolinium ion having high toxicity in a living body, since the number of carboxy groups, which can chelate gadolinium ion, is decreased.
Contrary to the above method, another method has been developed (Patent Documents 1 to 4 and Non-patent Documents 1 to 4). In the method, a substituent for improving a retention property in blood and the like is bound to the methylene group located between the carboxyl group and the amino group. The methylene group plays no role in the chelate to a gadolinium ion.
However, the target compound cannot be efficiently produced in the above conventional methods.
Specifically, for example, in the method described in Patent Document 3, Non-patent Documents 1, 3 and 4, a low-molecular compound having a substituent for improving a retention property in blood and the like or a precursor substituent thereof is reacted with other amine compounds and carboxy compounds to synthesize a DTPA derivative as the following scheme.

However, such a synthetic process requires many steps, and has problems of high cost and environmental deterioration. For example, a lot of waste substances such as halide ion salt are produced.
In Example described in Patent Document 4, a DTPA derivative having a reactive substituent is used as a starting compound, but the production method thereof is not explicitly described. The derivative may be probably produced by a similar method to the above.
In the method described in Patent Documents 1 and 2 and Non-patent Document 2, a substituent is bound to a DTPA ester in the presence of a base.
However, it is difficult according to the method to bond a substituent to an intended position only. In addition, a side reaction by which multiple substituents are bound is induced. Therefore, the yield of the target compound is low.